1. Field of the Invention
The present invention relates to a method for producing an electronic device such as an organic electroluminescent device (hereinafter, appropriately referred to as an “organic light-emitting diode (OLED)”), which is a self-luminous device used as an organic thin-film solar cell, a transistor device, a display device for displays, and a device for lightings, and to a coating solution suitable for the production method.
2. Description of the Related Art
Methods for producing an electronic device using various coating methods have been studied actively for achieving cost reduction, larger area devices, and flexibility. However, the requirement for using the coating methods is that a primary coating is insoluble in a solvent applied thereon.
For example, to produce a transistor device, a protective film is stacked by a coating method after the application of an organic semiconductor layer. In this case, the primary coating film (organic semiconductor layer) must be prevented from being dissolved in a solvent applied thereon.
Alternatively, to produce an organic thin-film solar cell, different or the same organic semiconductor materials are stacked. For example, p-type and n-type organic semiconductors are stacked by a coating method, or a mixed layer of p-type and n-type organic semiconductors is introduced into p-n junction. In such a case, the primary coating film must be prevented from being dissolved in a solvent applied thereon, or the interface between the layers must be controlled.
Next, an organic light-emitting diode (OLED) will be described as a specific example.
Research and development have been conducted actively on organic light-emitting diodes (OLED) since the report by Tang and VanSlyke (Non-Patent Document 1). In general, the organic light-emitting diodes (OLED) are divided into two groups, low-molecular-weight and high-molecular-weight organic matters. A film of the low-molecular-weight organic matter is generally formed by a method using vacuum vapor deposition. The use of the vacuum vapor deposition can easily prepare a multilayer structure of organic layers having independent functions and can produce a high-performance organic light-emitting diode (OLED). Recently, low-molecular-weight organic matter that exhibits excellent performance in an organic light-emitting diode (OLED) has been synthesized and has reached a practical level. However, the vapor deposition method performs film formation in a vacuum. Therefore, it is difficult to achieve larger area devices, and material utilization efficiency is insufficient.
By contrast, the high-molecular-weight organic matter can be produced by a method using various coating methods. Examples of the various coating methods include casting, spin-coating, ink jet, dipping, spraying, and printing methods. These coating methods can perform film formation at atmospheric pressure and easily achieve larger area devices as compared with the vapor deposition method. Moreover, material utilization efficiency is also high. Therefore, these methods are advantageous in terms of cost.
However, performance improvement using a multilayer structure requires selecting a solvent that does not dissolve therein the organic layer as a primary coating as a solvent for a coating solution applied later. Thus, it was difficult to prepare a multilayer structure due to constraints such as the limited selection of materials that can be used.
A known technique for stacking layers of organic matter involves performing insolubilization treatment using crosslinking (see e.g., Patent Document 1). However, this technique has constraints such as possible complicated production steps and a need to introduce a crosslinking group.
Alternatively, a technique often used involves stacking layers by a coating method using PEDOT/PSS, which is high-molecular-weight organic matter used in a water dispersion system, as a hole injection material and using high-molecular-weight organic matter as a luminescent layer material. However, this method is based on a water system, and a variety of ionic impurities might possibly influence the reliability of the organic light-emitting diode (OLED). Moreover, according to a report, the diffusion of an S or SO3 group from PEDOT reduces luminescence efficiency (Non-Patent Document 2).
In recent years, study has been made to apply low-molecular-weight organic matter to a coating method. In the formation of a multilayer structure, an organic layer of low-molecular-weight organic matter as a primary coating, as with high-molecular-weight organic matter, is soluble in a solvent for a coating solution applied later. Thus, it was difficult to prepare a multilayer structure.    [Patent Document 1] JP2000-077185A    [Non-Patent Document 1] C. W. Tang and S. A. VanSlyke: Appl. Phys. Lett., 51, p. 913 (1987)    [Non-Patent Document 2] “Organic EL Handbook”, published by Realize Science & Engineering Center Co., Ltd. 2004, p. 260
An object of the present invention is to provide a method for producing an electronic device, which is effective for every organic matter including low-molecular-weight and high-molecular-weight organic matters and is capable of forming a multilayer structure by a coating method using the organic matter, and a coating solution suitable for the production method. For example, an object of the present invention is to provide a method for producing an organic light-emitting diode (OLED) and a coating solution suitable for the production method.